1. Field of the Invention
This invention relates to a device for optically reading images such as characters and patterns on card-like documents, and more particularly to an optical image reading device provided with a mechanism capable of stably transporting a given document in a proper forwarding direction regardless of the size of the document, so as to reliably read the image on the given document with a high accuracy.
2. Description of the Prior Art
Image reading devices such as an image scanner and a card reader for optically reading images including characters and/or patterns on documents of various kinds such as a name card, plastic card, post card and envelope and converting image information thus read into electric signal data. Generally, the image reading device of this sort is provided with optical image reading means for optically scanning the image surface of a given document to obtain the corresponding image information, and a mechanism for transporting the document to scan the document with the optical image reading means. In order to automatically feed the document, the image reading device further calls for means for guiding the document, and means for moving the document along the guiding means toward the image reading position at a constant speed.
In the case where a demand is felt for the greater accuracy in performing an optical character reading (OCR) or other specific image processing, the document must be moved at a constant speed in the forwarding direction (sub-scanning direction) and stabilized in the width direction (main scanning direction). Namely, because the resolution and image reading accuracy of the device depend upon the performance of transporting the document, high stability in transporting the document is required.
For example, Japanese Patent Application Public Disclosure No. HEI 3-235470 discloses an image inputting device having a document feeder for forwarding a document along an image reading portion. The image reading portion of this conventional document feeder is formed in the shape of a tunnel having an entrance opening and an exit opening, through which the document is moved along one guide rib disposed on one side of a document passage. That is to say, the document is pushed by hand toward the guide rib when being forwarded to the tunnel-like image reading portion to be subjected to image reading. Since the tunnel-like image reading portion is closed on its both sides, the document having a width equal to the inner width of the image reading portion may be stably forwarded along the image reading portion without wobbling widthwise (in the main scanning direction) as guided by the both sides thereof. Whereas, when the document to be forwarded through the tunnel-like image reading portion is smaller in width than the image reading portion, the document cannot be forwarded stably. It is a matter of course that the document feeder having the tunnel-like image reading portion cannot deal with the document larger than the width of the image reading portion, otherwise it should be increased in size so as to enable handling of the large document.
In Japanese Utility Model Appln. Public Disclosure No. HEI 1-108659, there is disclosed another image reading device having a document passage which is open on one side thereof so that a document of large size can be dealt with. Namely, the document passage is encompassed by a casing formed in the shape of a generally square bracket ([) having an opening on its one side, so that even a large document can be passed therethrough. However, it is possible for the image reading device having the square bracket-shaped casing to stably deal with the document of small size as is similarly experienced in the aforesaid tunnel-like image reading portion, because the document passing through the image reading device is not positively held. The document advancing freely along the document passage will irregularly wobble widthwise, resulting in decrease in reading accuracy.
Accordingly, the document has to be practically held with finger tips pressed lightly toward an innermost guide surface, which is one of walls defining the document passage, when being sent through the image reading device having the one side opening. The work of manually sending the document along the document passage requires habituation to move the document accurately at a constant speed, but it is either impossible or very difficult. Besides, the conventional device having the one side opening cannot assure stability of transporting a document which is smaller in width than the document passage and difficult to hold by hand in passing through the document passage.
It goes without saying that use of a high-performance controlling system may make it possible to transport and optically scan a document of any size with a high accuracy, but disadvantageously, such a device inevitably turns out to be expensive and susceptible to mechanical trouble during service and cannot be used practically.